So I posted a thread 3 weeks ago (here's a link: Introducing New (male) Guinea Pig - Advice Needed) and just wanted to make an update thread about my current situation as a warning and advice for anyone that is thinking about making a trio 
After first introduction and separating our piggies (there was a fight), we tried again a week later. The introduction was terrible, after 5-10 minutes there was a fight between ALL OF THEM. So not just two piggies but three of them. We separated them immediately. Unfortunattely our alpha piggie (Romeo) was lightly wounded (by the new piggie), nothing serious just scratches, but it was really stressfull for us and for him.
We had to bath our older piggies because the new piggie (Bigos) was marking all of the area during the introduction and they smelled like him and it was making them nervous.
After the bath and cleaning Romeo's scratches we put them back in the cage. Bigos was already in his cage, resting.
For the first week everything was normal and calm, but then (a week ago) Romeo and Pogo started to act really agressive towards each other, there was a lot of teeth chattering, jumping, squeeking, mounting, yawning etc. This was really stressfull and hard to watch, because we felt guilty and we were really scared, constantly checking if everything's fine, there were some nights when we couldn't sleep because they were so loud.
We decided to not separate them unless there is a real fight. And by not separating them I mean really leaving them be, don't pet them, don't stress them, don't take them out. Just feed them, let them figure out their dominance and watch them.
Today the situation is way better, they are much calmer, sometimes we can hear some teeth chattering, but it's not that loud and it's like 5 seconds long, they eat together, sleep together and popcorn a little
Bigos is happy, gratefull, well fed, very vocal and lazy piggie
We will keep him alone for now, especially because he is in his puberty right now.
I wanted to post this thread to thank you, guys, you were so helpfull all the time and so smart and expierenced, there's no other forum like this one
Also I want to warn anyone who is thinking about getting another boar to an already bonded herd - it's stressfull and there really is a probability that you will destroy the bond your piggies already have - and it's painfull to watch, believe me!
We are lucky overall, we have three happy piggies, but if I knew then what I know now, I wouldn't put them and us through all of this stress.

After first introduction and separating our piggies (there was a fight), we tried again a week later. The introduction was terrible, after 5-10 minutes there was a fight between ALL OF THEM. So not just two piggies but three of them. We separated them immediately. Unfortunattely our alpha piggie (Romeo) was lightly wounded (by the new piggie), nothing serious just scratches, but it was really stressfull for us and for him.
We had to bath our older piggies because the new piggie (Bigos) was marking all of the area during the introduction and they smelled like him and it was making them nervous.
After the bath and cleaning Romeo's scratches we put them back in the cage. Bigos was already in his cage, resting.
For the first week everything was normal and calm, but then (a week ago) Romeo and Pogo started to act really agressive towards each other, there was a lot of teeth chattering, jumping, squeeking, mounting, yawning etc. This was really stressfull and hard to watch, because we felt guilty and we were really scared, constantly checking if everything's fine, there were some nights when we couldn't sleep because they were so loud.
We decided to not separate them unless there is a real fight. And by not separating them I mean really leaving them be, don't pet them, don't stress them, don't take them out. Just feed them, let them figure out their dominance and watch them.
Today the situation is way better, they are much calmer, sometimes we can hear some teeth chattering, but it's not that loud and it's like 5 seconds long, they eat together, sleep together and popcorn a little

Bigos is happy, gratefull, well fed, very vocal and lazy piggie

I wanted to post this thread to thank you, guys, you were so helpfull all the time and so smart and expierenced, there's no other forum like this one

Also I want to warn anyone who is thinking about getting another boar to an already bonded herd - it's stressfull and there really is a probability that you will destroy the bond your piggies already have - and it's painfull to watch, believe me!
We are lucky overall, we have three happy piggies, but if I knew then what I know now, I wouldn't put them and us through all of this stress.